Production of purified uranium



Jan. 26, 1960 L.. BURRls, JR., ETAI- PRODUCTION- OF' PURIF'IED URANIUM 2 Sheets-Sheet 1 Filed June 11, 1957 TESL Jan. 26, 1960 l.. BURRls, JR., ET AL 2,922,711

PRODUCTION oF PURIFIED URANIUM' 2 Sheets-Sheet 2 `Filed June l1, 1957 INVENToRs ttorzzy James B. -Jfz zgfLtoJL Harold Feder By-'KMA Qmzmfm Uran i um (T m le 8.. 2 e z new@ @,mww 0. w

jill

nite ates 'Patenti 2,922,71 Patented Jan. 26, 1960 lice Leslie Burris, Jr., James B. Knighton, and Hai-old M. Feder, Orland Park, Ill., assignors to the United States of America as represented by the United States Atomic Energy Commission Application June 11, 1957, Serial No. 665,105'

Claims. (C1. 715-841) This invention relates to a pyrometallurgical method for producing purified uranium metal suitable for use as fuel in nuclear reactors.

Pyrometallurgical methods hold great promise for the future in processing fuels for nuclear reactors. Because such methods eliminate many steps required in conventional chemical methods and because the methods lend themselves readily to continuousand remote operation, these methods will undoubtedly be widely used in the future. Y

It is accordingly an object of the present invention to develop a method for producing uranium of a high degree of purity.

Another object of the present invention is to provide a method for processing spent reactor fuel elements.

Still another object of the present invention is to provide a method for reducing binary compounds of uranium to metallic uranium.

'Ihese and other objects of the present invention are satisfied by our novel method for the purification and production of uranium in which the uranium is dissolved in zinc, an intermetallic compound of uranium and zinc is crystallized from the zinc solution by cooling, the crystals are separated from the solution, and the intermetallic compound decomposed by vaporizng the zinc therefrom. If the uranium to be purified is originally present as a binary compound such as U02 or UF4, the binary can be reduced to metallic uranium simultaneously to the purification thereof by employing a zinc-magnesium alloy to dissolve the uranium.

composition was prepared; I This alloy simulates nuclear reactor fuel in which there has been a build-up of fission l products;

Percent Uranium 91.46 Rutheium 2.97 Molybdenum 3.73 Zirconium 1.03 Palladium'1 .8l

2 the ii'se of zinc. For example, tantalum or reciystallized alumina may be used. A

To` sliow the purification obtained by fractional crystalliz'atio'ri from a zinc solution, an alloy of the following 1 Used as rhodium equivalent.

This alloy was dissolved in zinc at a temperature of approximately 800 C. andthe solution cooled by decrev ments. An analysis of the solution was made after every The invention is useful both in the production of I uranium metal from ores and in the processing of spent fuel elements to recover the uranium therefrom.

The invention will first be described in relation to the purification of metallic uranium such as that contained in spent fuel elements. As is well-known, separation of the ssion products from the uranium represents a problem of continuing interest to those skilled in the art.

The invention is of particular importance because separation of uranium from relatively non-oxidizable compounds, such as ruthenium and molybdenum, is obtained. For this reason the inventiony may be 4used along with the oxide dressing process described in U.S. patent application Serial No. 574,051 of Harold M. Feder and Norman R. Chellew, now Patent No. 2,822,260, which separates uranium effectively from other fission products.

According to the invention, the impure uranium is dissolved in zinc at about 800 C.; the resulting solution is cooled to about 500 C. and held at this temperature until fractional crystallization of an intermetallic compound of uranium and zinc having the formula UZng occurs; the crystals are then separated from the remaining zinc by filtration through a graphite filter having 90-micron pores; and the intermetallic compound is vaporized at atmospheric pressure, at a temperature of 945 C. Additional recrystallizatons may be performed v if necessary to remove all fission products.

The equipment employed to carry out the method may be made of most refractory materials compatible with decremental cooling. The results follow:

Table I Temp., C. U Ru Mo Pd Zr These results are plotted on Fig. 1 of the drawing. Reference to this figure shows clearly that substantially all the palladium remains in solution as the temperature is reduced. Likewise that part of the molybdenum which went into solution at 800 C. remained in solution aft 525 C. Therefore uranium can be separated from these elements by fractional crystallization. Also about onehalf the zirconium remains in solution and purification to this degree can be obtained by a single fractional crystallization. The solubility of rutheniumV on theother hand` decreases as the temperature is reduced. and the ruthenium therefore remains with the uranium.

A second recrystallization can be employed to separatel l the uranium vand ruthenium. Table lII and Figure y2 i plotted therefrom shows how this isr possible. Fi'gure v 2 includes points obtained yboth in run25 and in run '77.

Y Table I1 [Run #25-coo1ing rate=50 C./hour.]

Temp., C. U Ru [Run #77-coo1ing rate=25 C./hour.]

l These points disregarded-analytical error.

Betweenthe temperatures of 640 C. and 500 C. the solubility of ruthenium'in zinc in the presence of uranium does not drop nearly as fast as the solubility of uranium drops. Thus, there is a change in the behavior of ruthenium over that shown in Figure 1 Where other elements are present, and rutlienium can be separated from uranium by recrystallization from a ternary system of uranium, ruthenium, and zinc.

The invention will next be described with relation to the simultaneous reduction and purification of binary compounds of uranium such as U02, U03, and UF4.

Here again the invention may be applied to the process-V ing of spent fuel materials or to the processing of uranium compounds obtained during a process for the beneficiation and purification of ores. The reduction is according to the'following equations:

This aspect of the invention differs from that already described only in the fact Vthat an alloy of zinc and magnesium is used as the solvent. Thus uranium oxide, for example, is added to an alloy of zinc and magnesium at a temperature of not more than 850 C., and the mixture is stirred and retained at this temperature until the reaction is substantially completed. to 500 C., the crystals of UZn9 lteredlol, andthe UZn9 decomposed as before. An example illustrating this reduction follows:

187.5 grams Zn and 12.5 'grams Mg were heated-t0 800 C. ina furnace purged withargon. 11.3 grams of U-mesh U02 -were added, and the mixture was heated for an hour' and one-half with mixing. Therrhfot solution was` ltered through grade 40 porous graphite (90-micron pore size) and the ltrate was permitted 'to solidify. Examination `of a section of the ingot indicated the presence of UZng. from analytical results that at least 56 percent of the U02 was reduced. f Y

An'additional test was made to show the reduction of U03 with zine-12 w./o. magnesium alloy. 512.4 grams of an ingot prepared from 515.5 grams zinc and 32.5 grams magnesium was combined with 28.6 grams magnesium and 12.6'grams 10Q-mesh U03. This was heated at 800 C. for four hours with mixing with a tantalum stirring rod; After cooling the ingot showed evidence of good reduction. No unreacted U03 was evident under microscopic examination and large crystals of UZng were observed at the bottom of the ingot.

An'additional experiment was made to demonstrate the reduction of UF4. A mixture of 487.4 grams Zn-Mg ingot (6 percent Mg), 12 grams magnesium, 135 grams zinc, and 32.8 grams UF., was heated at 750 C. for one hour in an atmosphere of helium. The hot solution was then filtered. Examination of the ingot obtained on cooling indicated that the uranium had been reduced to UZng.

It was determined by calculationV It is thus clear that binary compounds of uranium can be reduced and simultaneously puried by fractional l crystallization from zinc employing magnesium as re- It is thenY Acooled ductant. It is believed that othersolvents such as cadium can be employed, and that similar processes Ycan be developed for the purification of plutonium.

It will be understood that this invention is not to be limited to the details given herein, but that it may be modified wihin the scope of the appended claims.

What is claimed is:

1. A pyrometallurgical method for the reduction of uranium compounds to metallic uranium comprising adding the uranium compounds to a'melt of an alloy of zinc and magnesium, cooling the mixture until UZn9 crystallizes therefrom, separating the VUZns, from Vthe liquid zinc, anddistilling zinc from the uranium to-pro duce metallic uranium.

2 A method according to claim 1 in which the uranium compounds are added at 800 C. and UZng is crystallized from' the solution at 500 C.

3. A `method according to claim l in which the alloy is 6 percent Mg and 94 percent Zn.

4. A method according to claim 1 in which a plurality of fractional crystallizatons are employed.

. 5. A method according to claim 2 in which the uranium compound' is U02.

6. A method according to claim 2 in which the uranium compound is U03.

7. A method according to claim 2 in which the uranium compound is UF4.

l8. A pyromertallurgical method for the purification of uranium metal from used nuclear reactor fuel elements comprising dissolving the fuel element in Zn, cooling the solution until UZng crystallizes therefrom, separating the UZn9 from the liquid zinc thereby removing rssion products from the uranium, and distilling the UZng to re move zinc therefrom thereby producing pure uranium metal.

9. A method according to claim 8 in Vwhich the uranium compounds are dissolved at 800 C. and UZng is crystallized from the solution at 500 C.

10. A method according to claim 8 in which a plurality of fracitional crystallizations are employed.

References Cited in the tile of this patent UNITED STATES PATENTS OTHER REFERENCES Bureau of Mines Report of Investigations, 5007, pages 16, 17.

Chiotti et al.: Journal of Metals, January 1957,' pages Y 

1. A PYROMETALLURGICAL METHOD FOR THE REDUCTION OF URANIUM COMPOUNDS TO METALLIC URANIUM COMPRISING ADDING THE URANIUM COMPOUNDS TO A MELT OF AN ALLOY OF ZINC AND MAGNESIUM, COOLING THE MIXTURE UNTIL UZN9 CRYSTALLIZES THEREFROM, SEPARATING THE UZN9 FROM THE LIQUID ZINC, AND DISTILLING ZINC FROM THE URANIUM TO PRODUCE METALLIC URANIUM. 